Superfocusing Effect Research Articles

Generalized Wavefront Manipulation: Dual-Foci Superfocusing with Hybrid-Magnitude Evanescent Modes and Terahertz Space-Division Multiplexing

To distinctively identify two objects at the deep-subwavelength scales requires sharp superfocusing to overcome the diffraction limit. However, conventional superfocusing effect and information-carrying capacity are limited by the focal length and single focusing field (only electric or magnetic field), which are hard to be apparently improved. Here, we introduce the concept of “dual-foci superfocusing”, an advanced focusing form that can provide either one or two focusing spots, simultaneously converging both electric and magnetic fields and presenting an effect of electromagnetostatic space-division multiplexing. The physical mechanism of the dual-foci superfocusing is analyzed and synthesized by an original theory of shaping functional fields using hybrid-magnitude evanescent modes. Through a terahertz plasmonic array, the dual-foci superfocusing is numerically demonstrated, whose metric (ratio of focal length to focusing-spot size) is sufficiently improved from traditional 1–1.8 up to 2.8. The proposed methodology could be exploited as a platform to investigate the novel concurrent characteristics of superfocusing and might represent an important step toward the development of beam manipulation and sophisticated holography.

First experimental observation of array of photonic jets from saw-tooth phase diffraction grating

Modern materials science and technologies based on the advanced lithographies, nanophotonics and electronics use the effect of super-focusing of light based on photonic nanojet (PNJ) array produced by close packing of spherical microparticles. To control the key parameters of photonic jets array the phase grating may be applied. For the first time the experimental direct imaging of the spatial and amplitude features of PNJ ensemble formation near the shadow surface of a saw-tooth phase diffraction grating is presented here for 3 different wavelengths: 405, 532, and 761 nm. It is shown that the PNJs array from such phase diffraction gratings, in contrast to microparticle self-assemblies, produces an array of PNJs with controlled characteristics: the length, width, and focal length of PNJ. They are changed in proportion to the incident wavelength. For the designed wavelength of , the length of PNJ is , the beam waist is and the focal length is equal to .

Publisher's Note: Experimental evidence of the superfocusing effect for axially channeled MeV protons [Phys. Rev. B94, 075415 (2016)

Publisher's Note: Experimental evidence of the superfocusing effect for axially channeled MeV protons [Phys. Rev. B<b>94</b>, 075415 (2016)

Experimental evidence of the superfocusing effect for axially channeled MeV protons

Sub-\AA{}ngstr\om focusing of megaelectronvolt (MeV) ions within axial channels was predicted over 10 years ago, but evidence proved elusive. We present experimental angular distributions of axially channeled MeV protons in a 55-nm-thick (001) silicon membrane through which multiple scattering is negligible. Fine angular structure is in excellent agreement with Monte Carlo simulations based on three interaction potentials, providing indirect evidence of the existence of the superfocusing effect with flux enhancement of around 800 within a focused beam width of $\ensuremath{\sim}20\phantom{\rule{0.16em}{0ex}}\mathrm{pm}$.

Fundamental laser modes in paraxial optics: from computer algebra and simulations to experimental observation

We study multi-parameter solutions of the inhomogeneous paraxial wave equation in a linear and quadratic approximation which include oscillating laser beams in a parabolic waveguide, spiral light beams, and other important families of propagation-invariant laser modes in weakly varying media. A "smart" lens design and a similar effect of superfocusing of particle beams in a thin monocrystal film are also discussed. In the supplementary electronic material, we provide a computer algebra verification of the results presented here, and of some related mathematical tools that were stated without proofs in the literature. We also demonstrate how computer algebra can be used to derive some of the presented formulas automatically, which is highly desirable as the corresponding hand calculations are very tedious. In numerical simulations, some of the new solutions reveal quite exotic properties which deserve further investigation including an experimental observation.

Multi-parameter laser modes in paraxial optics

By computer algebra methods we study multi-parameter solutions of the inhomogeneous paraxial wave equation in a linear and quadratic approximation which include oscillating laser beams in a parabolic waveguide and spiral light beams in weakly varying media. A similar effect of superfocusing of particle beams in a thin monocrystal film is also discussed.

Acoustic superfocusing by solid phononic crystals

We propose a solid phononic crystal lens capable of acoustic superfocusing beyond the diffraction limit. The unit cell of the crystal is formed by four rigid cylinders in a hosting material with a cavity arranged in the center. Theoretical studies reveal that the solid lens produces both negative refraction to focus propagating waves and surface states to amplify evanescent waves. Numerical analyses of the superfocusing effect of the considered solid phononic lens are presented with a separated source excitation to the lens. In this case, acoustic superfocusing beyond the diffraction limit is evidenced. Compared to the fluid phononic lenses, the solid lens is more suitable for ultrasonic imaging applications.

Diffractive lens design for optimized focusing.

We utilized nonlinear optimization to design phase-only diffractive lenses that focus light to a spot, whose width is smaller than that dictated by the far-field diffraction limit. Although scalar-diffraction theory was utilized for the design, careful comparisons against rigorous finite-difference time-domain simulations confirm the superfocusing effect. We were able to design a lens with a focal spot size that is 25% smaller than that formed by a conventional lens of the same numerical aperture. An optimization strategy that allows one to design such lenses is clearly explained. Furthermore, we performed careful simulations to elucidate the effects of fabrication errors and defocus on the performance of such optimized lenses. Since these lenses are thin, binary, and planar, large uniform arrays could be readily fabricated enabling important applications in microscopy and lithography.

Classical imaging theory of a microlens with super-resolution

Super-resolution in imaging through a transparent spherical microlens has attracted lots of attention because of recent promising experimental results with remarkable resolution improvement. To provide physical insight for this super-resolution phenomenon, previous studies adopted a phenomenological explanation mainly based on the super-focusing effect of a photonic nanojet, while a direct imaging calculation with classical imaging theory has rarely been studied. Here we theoretically model the imaging process through a microlens with vectorial electromagnetic analysis, and then exclude the previously plausible explanation of super-resolution based on the super-focusing effect. The results showed that, in the context of classical imaging theory subject to the two-point resolution criterion, a microlens with a perfect spherical shape cannot achieve the experimentally verified sub-100 nm resolution. Therefore, there must be some other physical mechanisms that contribute to the reported ultrahigh resolution but have not been revealed in theory.

Spiral laser beams in inhomogeneous media

Explicit solutions of the inhomogeneous paraxial wave equation in a linear and quadratic approximation are applied to wave fields with invariant features, such as oscillating laser beams in a parabolic waveguide and spiral light beams in varying media. A similar effect of superfocusing of particle beams in a thin monocrystal film, harmonic oscillations of cold trapped atoms, and motion in magnetic field are also mentioned.

The wave theory of infinite and finite Veselago lenses

The results of investigation of the problem of cylindrical wave scattering by infinite and finite Veslago lenses are discussed. The problem is considered in a rigorous electrodynamic formulation. It is shown that the superfocusing effect is impossible in an ideal Veselago lens. It is reported that the near-field structures observed in the case of diffraction of a cylindrical wave by an electrically thick lens and an electrically thin lens are substantially different.

Analysis of super focusing effect in tapered metal nano-structure with discrete dipole approximation method

The super focusing effect in tapered metal nano-structure is analyzed and discussed with discrete dipole approximation (DDA) method. Numerical result proves that the light field can be focused to nano-scale spot within this structure, and the relation between its super focusing ability and incident wavelength, structure’s vertex angle, thickness and metal’s dissipation is discussed, based on both analytic and numerical model. The results suggest that the electric field can be enhanced decades even hundreds times with properly designed parameters, which leads to a significant super focusing effect of light field. It also proves that the metal nano-structure can be modeled by DDA method with good simulation precision and speed.

Superfocusing of channeled protons and crystal rainbows

This study is devoted to the effect of superfocusing of protons having the energy of 2 MeV in a 〈1 0 0〉 channel of a Si crystal. The analysis is performed by the theory of crystal rainbows. We analyze the superfocusing effect in the first rainbow cycle. The evolution of the spatial distribution of channeled protons is examined by the numerical solution of the proton equations of motion in the transverse position plane. We demonstrate that the superfocusing effect is a reduced crystal rainbow effect, in which the rainbow line comes to a point.

Adaptive laser beam projection on an extended target: phase- and field-conjugate precompensation

The problem of adaptive laser beam projection onto an extended object (target) having a randomly rough surface in an optically inhomogeneous medium (atmosphere) is analyzed. Outgoing beam precompensation is considered through conjugation of either the target-return wave phase or the complex field. It is shown that in the presence of "frozen" turbulence, both phase-conjugate (PC) and field-conjugate (FC) precompensation can result in a superfocusing effect, which suggests the possibility of achieving a brighter target hit spot in volume turbulence than in vacuum. This superfocusing effect is significantly more distinct for FC precompensation. In the quasi-stationary case (slowly moving turbulence or target), PC and FC beam control lead to enhanced intensity fluctuations at the target surface associated with intermittent formation and disintegration of bright target hit spots that sporadically attach to the extended target surface. This intensity fluctuation level exceeds intensity fluctuations in the absence of beam control and is higher for FC precompensation. In the nonstationary case, both PC and FC lead to an increase of beam width and centroid wander at the extended target surface compared with conventional projection of a collimated or focused beam.

Evidence of four-body contributions in the EXAFS spectrum of Na 2Co[Fe(CN) 6

This contribution deals with an EXAFS investigation of Na 2Co[Fe(CN) 6] at the K-edges of both Fe and Co. Na 2Co[Fe(CN) 6], a well known and easily prepared mixed hexacyanoferrate, is characterised by a unit cell containing linear “FeCNCo” chains where a super-focusing effect may be observed at the K-edge of both transition metals. The multiple scattering (MS) signal intensity associated with the linear chain (four body contribution) is comparable with the first shell contributions. The combined fit over two edges, the intensity of the MS signal and the large number of experimental points included in the fit, permit an accurate extraction of the structural parameters at the limit of the EXAFS sensitivity.

Influence of the focusing effect on XAFS in ReO 3, WO 3− x and FeF 3

The role of the focusing effect in the formation of X-ray absorption fine structure (XAFS) is considered for ReO 3, non-stoichiometric tungsten oxides WO 3− x and FeF 3 having the perovskite-type structure. Two cases are mainly discussed: (1) an admixture of rhenium and tungsten L 2-edge XAFS in ReO 3 and WO 3− x crystals to the one above rhenium and tungsten L 1-edge and (2) high-order superfocusing effect in Fe 0F 1Fe 2F 3Fe 4 atomic chain in iron K-edge XAFS of FeF 3 which is analysed using an ab initio multiple-scattering approach.